Colloidal Quantum Dots as Photocatalysts for Triplet Excited State Reactions of Organic Molecules

Triplet excited state chemistry has enabled a range of important organic transformations by accessing reaction pathways inaccessible to photoredox chemistry. Such photoreactions are triggered by triplet photosensitizers, which absorb visible-light photons and transfer the energy to the substrate or to a co-catalyst through triplet-triplet energy transfer (TT EnT). The most popular triplet photosensitizers, metal complexes and organic chromophores, have proven useful in a range of pericyclic reactions, bond dissociations, and isomerizations, but they have several characteristics related to their chemical and electronic structure that limit their selectivity, energy efficiency, and sustainability. This Perspective describes some ways that colloidal quantum dots (QDs) address the limitations of molecular photocatalysts for TT EnT-driven organic transformations. These sub-5-nm particles have the large catalytic surface area and electronic/optical tunability of homogeneous catalysts, and the easy separation and surface templating effects of heterogeneous catalysts. Their optical and electronic properties, small singlet-triplet energy splitting, narrow emission line widths, and high photostability enhance their performance as triplet photosensitizers. This Perspective describes these advantages in the context of published and ongoing investigations of TT EnT-driven reactions, and then highlights the advantages and challenges associated with using related emerging materials, specifically lead halide perovskite QDs and quasi-2D nanoplatelets, as photocatalysts for triplet excited state chemistry.